The present invention relates to a single-tube color television image pickup apparatus having a frame memory in which the beam deflection system's nonlinearity information is stored as a reference signal for synchronous detection of primary colors.
In single-tube color television image pickup apparatus, a color filter is affixed to the photoelectrical conversion target of the image pickup tube to optically modulate a color image focused thereon and generate a color multiplexed video signal from the target.
Various proposals have been made in the arrangement of color filter stripes and in the circuit configuration that enables detection of the primary color components from the multiplexed video signal. One such color filter comprises a plurality of successively arranged, recurrent groups of filter stripes of different colors. The color-multiplexed signal has a frequency inversely proportional to the intervals at which the recurrent groups of the stripes are arranged. The signal varies in phase with the relative values of the primary color components contained in the color image passing through the stripes of each group and in amplitude with the intensity of the primary color components. In a known image pickup system a pair of synchronous detectors are provided to detect two of the primary color components from such multiplexed signal synchronously with a reference signal whose frequency is equal to the carrier frequency of the multiplexed signal.
One shortcoming inherent in the synchronous detection system is that the nonlinearity of the vertical and horizontal deflection signals causes the carrier frequency of the multiplexed video signal to deviate from the frequency of the reference signal.
To overcome this nonlinearity problem, a color image pickup apparatus, as shown and described in Japanese Examined Patent Publication (Kokoku) No. 53-34854, includes a frame memory. Prior to image pickup operation, the tube's target is uniformly illuminated with light of a predetermined primary color and is scanned with a beam which is subject to the inherent nonlinearity of the deflection signals. The video output derived from this illumination is stored in the memory as the reference signal for the synchronous detectors. During image pickup operation, the stored information is repeatedly read out of the memory to synchronously detect the primary color components. Since the frequency of the stored reference signal varies as a function of the nonlinearity, the reference signal and video signal match in frequency and phase with each other at the inputs of the synchronous detectors.
However, the prior art system is still subject to deflection signal variations which arise from instabilities in the deflection system and is also subject to noise arising from electromagnetic interference from external circuits. Such disturbing factors cause beam-spot size and scanning speed variations which result in a frequency difference between the video signal and the reference signal.